This invention relates to the hot rolling of thin strip. It has particular, but not exclusive, application to in-line hot rolling of thin steel strip produced by a twin roll strip caster.
Recent developments in twin roll strip casting has enabled steel strip to be produced to thicknesses of the order of 5 mm or less. Such strip can be further reduced in thickness by reduction in an in-line hot rolling mill as it is produced by the caster. It has been found that when hot rolling strip to a thickness of the order of 3 mm or less significant defects can be generated in the strip due to crimping of the strip material on entry to the reduction rolls. Such crimping defects may be relatively minor and appear as curved lines seen on the surface of the strip. However, particularly when rolling very thin strip, the crimped parts of the strip may become folded over prior to rolling so that parts of the strip become heavily reduced and split with resulting very severe defects.
Crimping defects arise due to variations in the strip reduction across the width of the strip. Typically, the centre part of the strip will be subjected to higher percentage reduction than the strip edges depending on incoming strip profile. The centre parts of the strip will therefore be subject to lengthwise compression whereas the edges will be subjected to tension. This results in backwards slip of the central portion of the strip relative to the edges. The centre of the strip is thus pushed back relative to the edges to create buckling. The buckles are then rolled into the strip to create marks on the surface. In extreme cases the strip may completely fold over at the buckles and the folded over material is rolled in to produce severe defects. Variations in strip reduction across the strip width can be localised to small parts of the strip width. This can result in localised crimping of the strip. The extent of the crimping is related to the size of the difference in reduction across the strip width and the size of the strip width affected by the difference in reduction.
Crimping of the strip is affected by strip tension and in some applications it is possible to control crimping defects by using high strip tension. However it is not always feasible to use higher tension in very thin strip at high temperatures. It is also difficult to apply high tensions to strip exiting a strip caster since little or no tension can be applied to the strip exiting the casting mould. The strip may hang in a free loop and be in a low tensioned state prior to passing to an in-line rolling mill. The present invention provides a method and apparatus by which crimping defects can be substantially controlled without the need for high strip tension.
According to the invention there is provided a method of hot rolling thin steel strip comprising feeding the strip through a nip between a pair of work rolls and applying strip squeezing forces between the work rolls, wherein the strip is passed in advance of the work rolls around a guide roll which causes the strip to be wrapped onto one of the work rolls in advance of its entry into the nip between the work rolls.
Preferably the strip is wrapped around said one work roll through an angle of wrap of at least 10xc2x0. The angle of wrap may for example be in the range 20xc2x0 to 45xc2x0.
Preferably the work rolls are disposed one above the other. In that case said guide roll may be positioned to guide the strip upwardly from a horizontal path to the nip and then downwardly about the upper work roll in advance of the nip.
The invention also provides strip rolling apparatus comprising;
a pair of work rolls defining a nip between them to receive a strip to be rolled,
means to apply squeezing forces between the work rolls, and
a guide roll to guide a strip to the nip between the work rolls such that the strip is wrapped onto one of the work rolls in advance of its entry into the nip.
Preferably the work rolls are disposed one above the other and the guide roll is movable from a lowered position in which strip can pass above it in a horizontal path to the nip between the work rolls and a raised operative position in which it lifts the strip above said horizontal path to cause the strip to be wrapped onto the upper work roll in advance of the nip.